1. Field of the Invention
The present invention relates generally to improved techniques for establishing radio communications between a vehicle and a roadside network, and more specifically to such techniques for use in ITS (Intelligent transportation systems). Still more specifically, the present invention is well suited for effectively establishing radio communication links between an in-vehicle unit and a roadside unit in an ETC (electronic toll collection) system.
2. Description of the Related Art
In an effort to solve a variety of transportation problems, a broad range of diverse technologies, known collectively as ITS, have been proposed. Among others, the ETC system has been found extremely preferable to eliminate traffic congestion and backups on toll booths.
Before turning to the present invention, it is deemed advantageous to briefly describe, with reference to FIG. 1, a conventional technique that may be relevant to the present invention.
As shown in FIG. 1, an antenna 10 is provided high on a pole 12 that is located on a roadside. When a moving vehicle approaches a toll facility or booth (not shown), the vehicle is detected using an approaching vehicle detecting unit 14 that is usually comprised of two devices located on each side of a lane (as best shown in FIG. 3). In FIG. 1, two vehicles 16 and 18 are schematically illustrated, which carry respectively in-vehicle units 20 and 22 for establishing a two-way communication with roadside network through the antenna 10. Each of the units 20 and 22 is located on a dashboard and includes an antenna (not shown). However, the above-mentioned conventional technique suffers from the following problem. That is, if the vehicle 16, which is a usual sedan in this case, approaches the pole 12 (viz., toll booth) immediately following the vehicle 18 (viz., a truck with a tall cargo room), the radio communication between the in-vehicle unit 20 and the antenna 10 is undesirably blocked. Since the radio communication is made via a direct wave, the quality of data to be transmitted is deteriorated to such an extent that the data is not correctly exchanged.
Accordingly, what is desired is to establish radio communication links which are not blocked by a tall vehicle.